X-ray tube bearing life is critical to high performance x-ray tube operation. In an x-ray tube, the primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, while the balance is converted to thermal energy. This thermal energy from the hot target is conducted and radiated to other components within the vacuum vessel of the x-ray tube. As a result of these high temperatures caused by this thermal energy, the x-ray tube components are subjected to high thermal stresses that are problematic in the operation and reliability of the x-ray tube.
Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a vacuum vessel. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. As mentioned above, the electrodes comprise the cathode assembly that is positioned at some distance from the target track of the rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode may be stationary. The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or a tungsten alloy, or, in mammo tubes, the target track is generally made of molybdenum. Further, to accelerate the electrons, a typical voltage difference of 60 kV to 140 kV (20 kV to 50 kV in mammo tubes) is maintained between the cathode and anode assemblies. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance is contained in back scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel along a focal spot alignment path. In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed along the focal spot alignment path to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector or film, and an image is formed of the internal anatomy therein. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks, or to inspect the contents of luggage at airports.
Since the production of x-rays in a medical diagnostic x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. For example, the temperature of the anode focal spot can run as high as about 2700° C., while the temperature in the other parts of the anode may range up to about 1800° C. Additionally, the components of the x-ray tube must be able to withstand the high temperature exhaust processing of the x-ray tube, at temperatures that may approach approximately 450° C. for a relatively long duration. The thermal energy generated during tube operation is typically transferred from the anode, and other components, to the vacuum vessel.
The high operating temperature of an x-ray tube is problematic for a number of reasons. The exposure of the components of the x-ray tube to cyclic, high temperatures can decrease the life and reliability of the components. In particular, the anode assembly is typically rotatably supported by a bearing assembly. This bearing assembly is very sensitive to high heat loads. Overheating the bearing assembly can lead to increased friction, increased noise, and to the ultimate failure of the bearing assembly.
The choice of bearing assembly lubricant in x-ray tubes is currently very restrained because the lubricant must have a very low vapor pressure at high temperatures (i.e., at or above 400° C.) in order to maintain the vacuum level in the tube. Furthermore, the lubricant must not release any particles into the vacuum that could disturb the high voltage stability therein. Therefore, generally only solid lubricants can be used to lubricate the bearing assemblies in x-ray tubes. Typically, solid lubricants such as silver or lead are used to coat the surfaces of the bearing assemblies. Lead, however, has a low melting point and a high evaporation rate, and therefore is not typically used in bearing assemblies exposed to operating temperatures above 400° C. because the high vacuum may not be able to be maintained. Furthermore, x-ray tubes using solid lead lubricant in the bearing assembly are typically limited to shorter, less powerful exposures. Above 400° C., silver is usually the solid lubricant of choice. Silver allows for longer, more powerful exposures than lead. However, silver is not as preferable as lead because silver has many drawbacks. Silver is much harder than lead and therefore, increases the noise generated by the bearing assembly. Furthermore, silver tends to react with the bearing steel if it becomes too hot, causing grain boundary cracking and premature failure of the bearing. Silver also requires more starting and running torque than lead due to its lower lubricity. Instead of being forced to use solid lubricants such as silver and lead in x-ray tube bearing assemblies, it would be desirable to be able to use various other types of lubricants, such as for example, oil, grease, powder, liquid, wetting metal, or the like. However, this is not currently possible.
As there are presently no suitable x-ray tube bearing assembly systems that allow lubricants other than solid lubricants to be used, it would be desirable to have such systems that would allow for any suitable lubricant to be used, whether solid or not. There is a need for such systems to allow oil, grease, powder, liquid, wetting metal, and other suitable lubricants to be used in the bearing assemblies. Such systems would ideally utilize one or more liquid metal gaskets to prevent the vapor and particles that are generated in the bearing assembly from entering the vacuum portion of the x-ray tube. The liquid metal gaskets of such systems may comprise an internal plug filled with liquid metal, such as mercury, gallium, or a gallium alloy, and may also comprise a first seal and a second seal. Such systems may allow any suitable bearing assembly lubricant to be used, such as for example, oils, greases, powders, liquids, wetting metals, and the like. Many other needs will also be met by this invention, as will become more apparent throughout the remainder of the disclosure that follows.